The Mass Function of Cosmic Structures with Non-Spherical Collapse

Non-spherical dynamical approximations and models for the gravitational collapse are used to extend the well-known Press & Schechter (PS) approach, in order to determine analytical expressions for the mass function of cosmic structures. The problem is rigorously set up by considering the intrinsic Lagrangian nature of the mass function. The Lagrangian equations of motion of a cold and irrotational fluid in single-stream regime show that the shear, which is non-locally determined by all the matter field, is the quantity which characterizes non-spherical perturbations. The Zel’dovich approximation, being a self-consistent first-order Lagrangian and local one, is used as a suitable guide to develop realistic estimates of the collapse time of a mass clump, starting from the local initial values of density and shear. Both Zel’dovich-based \an\ and models and the homogeneous ellipsoidal model predict that more large-mass objects are expected to form than the usual PS relation. In particular, the homogeneous ellipsoid model is consistent at large masses with a Press & Schechter mass function with a lower value of the \dc
parameter, in the range 1.4$÷$1.6. This gives a dynamical explanation of why lower \dc\ values have been found to fit the results of several N-body simulations. When more small-scale structure is present, highly non-linear dynamical effects can effectively slow down the collapse rate of a perturbation, increasing the effective value of \dc. This may have interesting consequences on the abundance of large-mass high-redshift objects.